There are many Social Bookmarking tools available, but the basic premise is that they allow you to save online content (websites, files, etc) to your personal archives, tagged with whatever subject terms or key words you associate with that content. You set up subject folders and can often add your own comments to each bookmark. Most Social Bookmarking tools also make it easy for you to find content that other people have bookmarked, often based on whose archives are similar to yours. Many are entirely web based, which means you can access them from any computer, not just where you've saved things to your hard-drive or personal browser bookmarks.
I never surf the web without my Social Bookmarking account turned on, and now I never have trouble finding things I've read before! I use Furl. There are many different tools now available, but I find that Furl has the most features that I find useful.
Social Bookmarking is a huge new field and one that I am excited to help people learn to use as part of their daily high-powered information gathering practice.